Due to their excellent formability and favorable physical properties, plastics are used for a vast variety of goods including packaging materials, other articles of daily use, agricultural and civil engineering materials, and parts for household appliances and automobiles. However, plastics are now posing grave social problems because they are produced from petroleum, one of the natural resources of which the underground reserves are limited, and because used plastic products are difficult to dispose of.
Therefore, uses of biodegradable plastics, i.e. plastics derived from vegetables, which are free of these problems, are rapidly increasing these days. A typical such plastic is polylactic acid, which is a plastic obtained by polymerizing lactates, which are in turn obtained by fermenting corns or sugar beets.
While such polylactic acid is widely used for materials for nondurable consumer goods such as packaging materials and other daily necessities (see for example the below-identified References 1 and 2), it has been used for very few durable goods, such as parts of household appliances and automobiles, for which extremely high durability is required.
This is because neither polylactic acid itself nor any existing composition containing polylactic acid has flame retardancy enough to clear the lower limit values set under JIS (Japanese Industrial Standards) or UL (Underwriters' Laboratories) for parts of household appliances and automobiles.
However, the below-identified Reference 3 discloses a resin comprising a biodegradable plastic to which is added aluminum hydroxide powder or magnesium hydroxide powder to cause the biodegradable plastic to have flame retardancy.
Reference 1: JP patent publication 7-207041
Reference 2: JP patent publication 7-308961
Reference 3: JP patent publication 8-252823
But if the biodegradable plastic used in Reference 3 is an aliphatic polyester such as polylactic acid, a hydroxide such as aluminum hydroxide or magnesium hydroxide that is added to this biodegradable plastic tends to hydrolyze the biodegradable plastic especially during melt compounding or melt forming, thus markedly lowering the molecular weight of the resin. This in turn aggravates various physical properties, such as strength, of the resin composition or molded article obtained to such an extent that these resin composition and molded article are practically useless.
An object of the invention is therefore to provide a resin composition containing a lactic acid resin and having enough flame retardancy as well as a sufficiently large molecular weight, and an article formed by molding this composition.
The present invention provides a flame-retardant resin composition comprising a resin composition mainly comprising 100 parts by mass of a lactic acid resin, and 50-150 parts by mass of a surface-treated metallic hydroxide.
Since the metallic hydroxide is surface-treated, it will not directly contact the resin composition when mixed with the resin composition. This suppresses hydrolysis of the resin composition and thus the lowering of its molecular weight.
Addition of the surface-treated metallic hydroxide in the above-defined amount will cause the resin composition to have sufficient flame retardancy.
Thus, the injection-molded article according to the invention has sufficiently high flame retardancy, shock resistance and heat resistance, while substantially keeping the biodegradability of the lactic acid resin intact. Also, since the resin composition is scarcely hydrolyzed, its molecular weight is kept high.
By further adding a carbodiimide compound to this composition, the molecular weight of the injection-molded article will scarcely decrease even if it is actually used in a harsh environment such as a high-humidity, high-temperature environment.
By further adding an inorganic filler to the composition, dimensional stability of the flame-retardant injection-molded article according to the invention will improve.